KR20120076345A - High-strength high-toughness cast steel material and manufacturing method therefor - Google Patents

Abstract

An object of the present invention is to ensure high strength and high toughness even in large cast steels without implementing a manufacturing method requiring rapid cooling such as liquid immersion.
The high strength and high toughness cast steel of the present invention is 0.10 to 0.20 mass% of C, 0.10 to 0.50 mass% of Si, 0.40 to 1.20 mass% of Mn, 2.00 to 3.00 mass% of Ni, and 0.20 to 0.70 mass% of Cr. And Mo containing 0.10 to 0.50% by mass and further containing Fe and unavoidable impurities. The high strength and high toughness cast steel of the present invention is annealing the ingot having the composition at 1,000 to 1,000 ℃, quenching at 850 to 950 ℃, tempering at 610 to 670 ℃, then 610 as necessary It is prepared via a step of stress relaxation annealing below < RTI ID = 0.0 >

Description

High strength, high toughness cast steel and its manufacturing method {HIGH-STRENGTH HIGH-TOUGHNESS CAST STEEL MATERIAL AND MANUFACTURING METHOD THEREFOR}

FIELD OF THE INVENTION The present invention relates to high strength and high toughness cast steels suitable for large cast steel products having large wall thicknesses and complex shapes and weights in excess of one metric ton and that may be welded, and methods of making the same. .

As a cast steel material that can be welded having high toughness and high strength, SCW480, SCW550 and the like described in Japanese Industrial Standards are well known. In the past, steel materials disclosed in Patent Documents 1 to 4 have been invented.

The steel disclosed in Patent Document 1 is a pre-cured steel for molding for plastics, and has been subjected to aging hardening heat treatment after hot working of steel containing predetermined components. In the steel materials disclosed in Patent Document 2, high strength and high toughness are achieved by plastic working such as forging or rolling, or high strength and high toughness are water cooled or in heat treatment such as quenching or normalizing. It is achieved by cooling using a method that exhibits a high cooling effect such as oil cooling. In the steel materials disclosed in Patent Document 3, in order to secure mechanical properties, the average cooling rate is controlled at about 250 ° C./min during austenitizing treatment, and this average cooling rate is about 300 nm in thickness. It is a cooling rate comparable to water cooling for large cast steel products. In addition, Patent Document 4 discloses a production method in which a slab containing a predetermined component is cooled at a cooling rate of 0.5 ° C / sec or more between the solidification temperature of the slab and 1,000 ° C.

JP-A-2005-82814 JP-T-2004-514060 JP-A-2001-181783 JP-A-2000-26934

On the other hand, in cast steels, the application of water cooling or oil cooling to large products with large wall thicknesses and complex geometries and weights in excess of one metric ton, such as problems with crack initiation and steam pollutants due to thermal stress in the cooling stage, There are difficulties in terms of safety issues, so air cooling or fan cooling is carried out in heat treatments such as quenching and normalizing. If the cooling rate is as low as described above, there is a problem that it is difficult to ensure sufficient strength and toughness in the component range described in the SCW480 or SCW550 or in each of the patent documents mentioned above.

The present invention is designed to ensure high strength and high toughness in the above mentioned large cast steel products, and an object of the present invention is to provide a cast steel material and such a material which can have sufficient high strength and high toughness even by air cooling or fan cooling. It is to provide a method of manufacturing

The present invention relates to the following high strength and high toughness cast steels, and methods of making the same.

<1> High strength and high toughness cast steel is 0.10 to 0.20 mass% of C, 0.10 to 0.50 mass% of Si, 0.40 to 1.20 mass% of Mn, 2.00 to 3.00 mass% of Ni, 0.20 to 0.70 mass% of Cr And Mo containing 0.10 to 0.50% by mass and further containing Fe and unavoidable impurities.

<2> The product mass of the high strength and high toughness cast steel according to <1> is 1 metric ton or more.

<3> The high strength and high toughness cast steel according to <1> or <2> further contains V as 0.05 wt% or less as a composition component.

<4> The high strength and high toughness cast steel according to any one of <1> to <3> further contains N by 20 to 150 ppm by mass as a composition component.

<5> The high strength and high toughness cast steel according to any one of the above <1> to <4> is less than 0.01 mass% of Al, less than 0.01 mass% of Ti, less than 0.025 mass% of Sn, and 0.015 mass% of P. Less than 0.015 mass% as an unavoidable impurity.

<6> 0.10 to 0.20 mass% of C, 0.10 to 0.50 mass% of Si, 0.40 to 1.20 mass% of Mn, 2.00 to 3.00 mass% of Ni, 0.20 to 0.70 mass% of Cr, and 0.10 to 0.50 mass of Mo A method for producing a high strength and high toughness cast steel for an ingot having a composition containing% by weight and further containing Fe and unavoidable impurities, the method comprising an annealing step of performing a heat treatment at 1,000 to 1,000 ° C. ; A quenching step of conducting heat treatment at 850-950 ° C .; And a tempering step of performing a heat treatment at 610 to 670 ° C.

<7> The method for manufacturing the high strength and high toughness cast steel according to <6> further includes a stress-relaxing annealing step of performing heat treatment at less than 610 ° C. after the tempering step.

<8> In the method for producing a high strength and high toughness cast steel according to <6> or <7>, each of the annealing step and the quenching step includes a cooling step, wherein both cooling steps are cooled by liquid immersion. This is done at a lower cooling rate than when.

<9> In the method for producing a high strength and high toughness cast steel according to any one of the above <6> to <8>, the composition of the ingot is a condition containing V of 0.05% by mass or less and 20 to 150% of N At least one of the conditions containing ppm is further satisfied.

As described above, since the high strength and high toughness cast steel of the present invention has a specific composition, even in large cast steels, sufficient high strength and high toughness can be applied to liquids such as water cooling or oil cooling without the application of plastic working and upon quenching. Obtained by air cooling or fan cooling without performing cooling.

1 is a diagram showing a test material produced at the same cost as a large cast steel product and a location where various test pieces are sampled from the test material.
FIG. 2 is a graph showing the relationship between tensile strength and absorbed energy based on the results shown in Table 6. FIG.
3 is a graph showing the relationship between tensile strength and absorbed energy based on the results shown in Table 7.

Cast steel

In the following, an embodiment of the present invention will be described.

In the present application, when simply described as "%" and "ppm", "mass%" and "mass ppm" respectively mean.

The high strength and high toughness cast steel of the present invention (hereinafter also referred to as the cast steel of the present invention) is 0.10 to 0.20 mass% of C, 0.10 to 0.50 mass% of Si, 0.40 to 1.20 mass% of Mn, and 2.00 of Ni. To 3.00 mass%, Cr to 0.20 to 0.70 mass%, and Mo to 0.10 to 0.50 mass%, further containing Fe and unavoidable impurities. Furthermore, if necessary, V contains less than 0.05% and N contains one or both of 20 to 150 ppm.

The following will show the limited reasons for the composition of the present invention.

C (carbon): 0.10 to 0.20%

C is an element which improves strength and hardenability. On the other hand, when C is added in excess, it is difficult to obtain a predetermined toughness and the sensitivity to weld cracking also increases. In view of the above factors, the content of C is determined to be 0.10 to 0.20%. For this reason, the preferred lower limit is 0.12% and the preferred upper limit is 0.16%.

Si (silicon): 0.1 to 0.50%

Si is an element used as a deoxidizer and improving hardenability. On the other hand, the content is determined to be 0.10 to 0.50% because of increased segregation and excessive formation of non-metallic inclusions to reduce toughness when excessive Si is added. For this reason, the preferred lower limit is 0.20%, the preferred upper limit is 0.40% and the more preferred upper limit is 0.30%.

Mn (manganese): 0.40 to 1.20%

Mn is an element which improves strength and hardenability. On the other hand, when the content is less than 0.40%, the predetermined strength is not obtained. On the other hand, when the content exceeds 1.20%, the strength is too high to obtain the desired ductility and toughness, and temper embrittlement may also occur. Therefore, the content of Mn is determined to be 0.40 to 1.20%. For this reason, the preferred lower limit is 0.50% and the preferred upper limit is 1.00%.

Ni (nickel): 2.00 to 3.00%

Ni is an element which improves strength and hardenability and has an effect on improving low temperature toughness. On the other hand, Ni has the effect of lowering strength and toughness due to excessive addition, and there is also a fear of welding crack initiation. Furthermore, since Ni is an expensive element, it is preferable to suppress the amount added. In view of the above factors, the content of Ni is determined to be 2.00 to 3.00%. For this reason, the preferred lower limit is 2.20% and the preferred upper limit is 2.60%.

Cr (chrome): 0.20 to 0.70%

Cr is an element that improves strength and hardenability. Since the strength is improved by the carbon structure, the predetermined strength is not obtained when the content is low. On the other hand, its excess addition leads to deterioration in the viscosity. Therefore, the content of Cr is determined to be 0.20 to 0.70%. For this reason, the preferred lower limit is 0.40% and the preferred upper limit is 0.65%.

Mo (molybdenum): 0.10 to 0.50%

Mo is an element which improves hardenability and reduces temper embrittlement. On the other hand, excessive addition thereof leads to deterioration of weldability. Therefore, the content of Mo is determined to be 0.10 to 0.50%. For this reason, the preferred lower limit is 0.15% and the preferred upper limit is 0.25%.

The cast steel of this invention contains the following component component as needed.

V (vanadium): 0.05% or less

V can be contained as needed as an element which improves strength by precipitation hardening. On the other hand, it is an element that inhibits weldability, and the toughness thereof is considerably lowered by the excessive addition thereof. Therefore, in the case of containing V, the content is determined to be 0.05% or less. In order to be fully affected by precipitation hardening, it is preferable to contain in about 0.02% or more.

N (nitrogen): 20 to 150 ppm

N is inevitably contained, but has the effect of refining the crystal grains and forms nitrides such as V and the like and increases the yield strength. On the other hand, there is a concern that lowering of toughness can be caused by excessive precipitation of TiN. In order to ensure mechanical properties, the residual amount of 20 to 150 ppm is preferable, and 50 ppm as the lower limit and 120 ppm as the upper limit are more preferable.

Inevitable impurities

The cast steel of the present invention may further contain an acceptable amount of unavoidable impurities. It is desirable to limit the inevitable impurities Al, Ti, Sn, P and S contained in the cast steel of the present invention to within the specific amounts shown below. In addition to the inevitable impurities other than those described above, it is desirable to limit the content in order to improve the mechanical properties.

Al (aluminum): less than 0.01%

Al is an element to be added as an oxygen scavenger and has the effect of AlN formation during deoxygenation and heat treatment to prevent austenite grains from grain coarsening. On the other hand, in cast steel, since sand marks due to Al ₂ O ₃ , defect generation due to rock candy and the like are problematic, it is desirable to reduce their residual amount as much as possible. Therefore, amounts less than 0.01% are suitable.

Ti (titanium): less than 0.01%

Ti is an element which improves strength by precipitation of TiN. On the other hand, excessive precipitation of TiN can lead to lowered toughness. Since a certain degree of N contamination is inevitable in large cast steel products produced by casting in an air atmosphere, it is desirable to reduce the amount of Ti as much as possible in order to ensure high toughness, and therefore an amount of less than 0.01% is preferable.

Sn (Tin): 0.025% or less

Sn is an element that significantly lowers the toughness by adding more than 0.03%. In order to ensure high toughness, it is preferable to control the content to less than 0.025%, and more preferably to control the content to less than 0.01%.

P (phosphorus): less than 0.015%

S (sulfur): less than 0.015%

Although P and S are inevitably contained impurity components, P has a grain boundary boundary and S binds to Mn or the like to form inclusions, both of which have a function of lowering mechanical properties. In order to ensure mechanical properties, it is preferable to reduce the residual amount as much as possible, and a content of less than 0.015% is suitable.

Manufacturing method

Hereinafter, a method of manufacturing the cast steel of the present invention will be described.

Regarding the cast steel of the present invention, the cast steel (raw material) can be obtained by casting according to an existing method, wherein the casting method is not limited to a specific method.

Regarding the cast steel of the present invention, for example, after the molten raw material is melted according to the existing method and adjusted to the composition described above, the ingot is obtained by casting into a mold. Thereafter, heat treatment is carried out at an annealing step at 1,000 to 1,100 ° C., then heat treatment is performed at a quenching step at 850 to 950 ° C., then heat treatment is performed at a tempering step at 610 to 670 ° C., and If necessary, it is carried out in a stress-relaxing annealing step followed by heat treatment below 610 ° C., whereby the cast steel can be produced.

Annealing Step: 1,000 to 1,100 ° C

The annealing step is carried out for the purpose of alleviating the stress produced in the mold during casting and homogenizing the components produced during solidification, and heating is carried out at 1000 ° C or higher. On the other hand, heating is limited to a temperature range of 1,000 to 1,100 ° C because the grains become too rough and the toughness becomes low when the heating is performed at a temperature exceeding 1,100 ° C.

Quenching step: 850 to 950 ℃

Quenching and tempering are performed to ensure mechanical properties. In the quenching step, it is required to control the temperature above 850 ° C. in order to achieve the austenite single-phase state, but when the temperature exceeds 950 ° C. grain coarsening begins and the toughness is significantly lowered so that the temperature It is limited to a temperature range of 850 to 950 ° C.

Tempering step: 610 to 670 ℃

Because the temperature is significantly high, the tensile strength is lowered, and the toughness is lowered when the austenite phase is precipitated through reverse transformation, so the tempering step is required to be performed at 670 ° C or lower. Furthermore, when the tempering step is carried out at a significantly lower temperature, the balance between strength and toughness is worse and the toughness is lowered, so that the tempering step is preferably carried out at 610 ° C. or higher. Thus, the tempering step is limited to a temperature range of 610 to 670 ° C.

Incidentally, the heat-holding time in the annealing, quenching and tempering is determined depending on the thickness of the product, but it is preferable to maintain heating for at least 10 hours to achieve a sufficient effect.

Stress-Relieving Annealing Step: Below 610 ° C

The stress-relaxing annealing step is carried out for the purpose of mitigating the stresses generated in the structure welding and the repair welding, and is added after the tempering step if necessary. In order to fully show the stress-relaxing effect, it is necessary to carry out this step at a temperature as high as possible. On the other hand, if this step is carried out at the same temperature as the tempering temperature, the mechanical properties are affected, so it is preferable that this step be carried out below 610 ° C. Furthermore, the holding time is also determined by the welding amount, but in order to achieve a sufficient effect, it is preferable to maintain the temperature at 4 hours or more.

Further, according to the present invention, sufficiently high strength and high toughness when the cooling step is carried out at a cooling rate lower than that realized by liquid immersion in a treatment called austenitization treatment comprising an annealing step and a quenching step This can be obtained. As the cooling method at this cooling rate, for example, air cooling and fan cooling can be mentioned.

The cast steel of the present invention obtained by the above-described manufacturing method has high strength and high toughness. The material can be suitably used as a final product having a mass of at least 1 metric ton and having a maximum wall thickness of at least 100 nm.

The cast steel of the present invention is particularly suitable for cast steel products having a product mass of at least 1 metric ton, more preferably at least 5 metric tons, even more preferably at least 10 metric tons. Furthermore, it is also suitable for complex-shaped articles having a maximum wall thickness of 100 mm to 300 mm. On the other hand, the present invention is not limited to having a product mass or a maximum wall thickness corresponding to each of the above ranges.

Example

In the following, the present invention will be explained by comparing Examples of the present invention to Comparative Examples.

The components shown in Table 1 were melted in a vacuum induction melting furnace (hereinafter referred to as VIM) and cast into a sandpaper having a length of 240 mm, a height of 250 mm and a width of 90 mm to obtain an ingot. The ingots were cut into 80 mm long, 120 mm high, and 30 mm wide, and after cutting the ingots were kept at 1050 ° C. for 20 hours and cooled to 50 ° C./hour to undergo the annealing step. Next, this was kept at 890 ° C. for 20 hours, and then quenched by cooling at a rate of 300 ° C./hour. During quenching, the cooling rate accelerated the cooling rate following the fan cooling at a depth of 125 mm from the surface of the large cast steel product.

Furthermore, after holding at 610 ° C. for 20 hours, cooling was performed at a rate of 50 ° C./hour to carry out the tempering step, followed by holding at 600 ° C. for 6 hours, followed by cooling at 75 ° C./hour. An annealing step was performed. The annealing step promotes stress-relaxing annealing that relieves residual stress loaded by welding or the like.

Tensile test pieces and Sarphi impact test pieces were prepared from the cut ingot after heat treatment, and then tested. The tensile test was conducted with a test piece of JIS No. 14-A, and the Sarphi impact test was conducted with a test piece of JIS No. 4.

Furthermore, tensile test pieces and sarphi impact test pieces were prepared and tested from test materials prepared at the same cost as the large cast steel product having the components shown in Table 1.

1 shows where the test material and the tensile test piece and the sarphi impact test piece are sampled from the test material.

Tensile tests were conducted using tensile test pieces, confirming the decrease in tensile strength, 0.2 @ yield strength, elongation, and area. The test was conducted at room temperature.

Furthermore, the Sarphi impact test piece was used to test the Sarphi impact test piece and the absorbed energy was confirmed. The test was conducted at 0 ° C.

The test results for the ingot and the test material are shown in Table 2. Since structural materials requiring high strength and high toughness require a degree of strength and toughness, it is the goal of the individual mechanical features for large cast steel products to be evaluated for tensile strength of at least 620 MPa and absorbed energy of at least 75 J.

Furthermore, from the results shown in Table 2, no significant difference was observed in strength, but it was confirmed that the absorption energy of the test material was about 20 J smaller than the absorption energy in the ingot. Therefore, target values for small test materials were determined with tensile strength of at least 620 MPa and absorbed energy of at least 95J.

Furthermore, since the stress-relieving annealing temperature is 600 ° C., the annealing temperature was defined as 610 ° C. or more.

The following shows the test results determined from the individual components.

The comparative material components for which the amount of V changes are shown in Table 3. The components shown in Table 3 were melted in VIM and cast into sand molds of 240 mm long, 250 mm high, and 90 mm wide to obtain an ingot. The ingots were cut into 80 mm long, 120 mm high, and 30 mm wide, and after cutting the ingots were kept at 1050 ° C. for 20 hours and cooled to 50 ° C./hour to undergo the annealing step. Next, this was maintained at 910 ° C. for 20 hours, and then quenched by cooling at a rate of 300 ° C./hour. Next, after holding at 640 ° C. for 20 hours, tempering was performed by cooling at a rate of 50 ° C./hour, followed by holding at 600 ° C. for 6 hours, and cooling at a rate of 75 ° C./hour to give stress- Palliative annealing was performed.

The test results using the test material are shown in Table 4. As shown in the results, the strength is increased when V is contained in a very small amount, but the toughness is decreased. This is due to precipitation hardening induced by V, and thus this fact shows that excessive addition of V is prohibited in large cast steels.

Test material components in which the amounts of Mn and Ni are varied are shown in Table 5. The furrows shown in Table 5 were melted in VIM and cast into sand molds of length 240 mm, height 250 mm, and width 90 mm to obtain an ingot. The ingots were cut into 80 mm long, 120 mm high, and 30 mm wide, and after cutting the ingots were kept at 1050 ° C. for 20 hours and cooled to 50 ° C./hour for the annealing step. Next, this was kept at 890 ° C. for 20 hours, and then quenched by cooling at a rate of 300 ° C./hour. Next, after maintaining at 640 ℃ and 610 ℃ for 20 hours, the cooling was carried out at 50 ℃ / hour rate to perform tempering, and then maintained at 600 ℃ for 6 hours, and cooled at 75 ℃ / hour rate Stress-relaxed annealing was performed.

Test results using the test material are shown in Table 6. 2 shows the relationship between tensile strength and absorbed energy based on the results shown in Table 6. FIG. As can be seen from the results, the strength and toughness are increased when Ni is added to about 2.50% or less (steel materials 2 and 3 of the present invention), and the target strength and toughness can be obtained by adding 2.00 to 3.00% of Ni. On the other hand, when Ni was added up to 3.50% (Comparative Examples 3 and 4), both strength and toughness decreased inversely, which resulted in this case being considered excessive addition.

With regard to Mn, a suitable content was estimated by comparing Comparative Material 2 in Table 4 and inventive steels 2 and 3 in Table 6 above. That is, when about 1.80% of Mn is added, the strength is too strong to obtain the predetermined toughness. On the other hand, the target strength and toughness are obtained in the steel materials 2 and 3 of the present invention containing 0.50% to 1.00% Mn. On the other hand, considering the balance between strength and toughness, if Mn was further reduced, the target strength could no longer be obtained.

From the above results, the amount of Ni to be added was determined to be 2.00 to 3.00%, and the amount of Mn to be added was determined to be 0.40 to 1.20%.

Further, the steel material 3 of the present invention was quenched at a cooling rate of 50 ° C / hour, 300 ° C / hour, and 900 ° C / hour. Cooling rates of 50 ° C./hour and 900 ° C./hour promote cooling rates by furnace cooling and spray cooling, respectively, at a point 125 mm deep from the surface of the large cast steel product.

The test results of the tensile test and the Sarpy impact test using the test ingot obtained by quenching Steel 3 of the present invention at various cooling rates are shown in Table 7.

3 shows the relationship between tensile strength and absorbed energy based on the results shown in Table 7.

This confirmed the tendency for both strength and toughness to improve in the steel of the present invention as the cooling rate increased during quenching.

Although sufficient strength and toughness cannot be ensured by furnace cooling, it has been found that sufficient strength and toughness can be ensured when fan cooling and spray cooling are carried out.

From the above results, in the components of the steel materials 1 to 3 of the present invention, high strength and high toughness steels are not subjected to liquid cooling such as water cooling or oil cooling during heat treatment such as quenching and normalizing the large cast steel products. Confirmed.

Incidentally, although the increase in tempering temperature is effective for improving the toughness, the steels 1 to 3 of the present invention have a eutectic temperature of about 690 ° C., thus 670 ° C. is a tempering temperature which may fail in normal operation. Is the upper limit. In consideration of including these test results, a tempering temperature of 610 to 670 ° C is suitable.

While the invention has been described in detail and based on certain embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application claims priority with respect to Japanese Patent Application No. 2009-220750 for which it applied on September 25, 2009, The content is taken in here as a reference.

While sufficient high strength and high toughness can also be obtained by air cooling or fan cooling, the high strength and high toughness cast steels of the present invention are characterized in that liquid cooling, such as water cooling or oil cooling, is used in heat treatment such as quenching and normalizing. Particularly useful for large cast steel products, which are difficult to apply, have large thicknesses with a maximum wall thickness of 100 mm to 300 mm and complex shapes or have weights in excess of 1 metric ton.

Claims (9)

0.10 to 0.20 mass% of C, 0.10 to 0.50 mass% of Si, 0.40 to 1.20 mass% of Mn, 2.00 to 3.00 mass% of Ni, 0.20 to 0.70 mass% of Cr, and 0.10 to 0.50 mass% of Mo And a composition further containing Fe and unavoidable impurities. The method of claim 1,
The product has a mass of at least 1 metric ton, high strength and high toughness cast steel. The method according to claim 1 or 2,
A high strength and high toughness cast steel further comprising V as a composition component at 0.05 mass% or less. The method according to any one of claims 1 to 3,
A high strength and high toughness cast steel, characterized by further containing 20 to 150 ppm by mass of N as a composition component. The method according to any one of claims 1 to 4,
High strength and high toughness cast steels contain less than 0.01 mass% of Al, less than 0.01 mass% of Ti, less than 0.025 mass% of Sn, less than 0.015 mass% of P, and less than 0.015 mass% of S as an unavoidable impurity. Characterized by high strength and high toughness cast steels. 0.10 to 0.20 mass% of C, 0.10 to 0.50 mass% of Si, 0.40 to 1.20 mass% of Mn, 2.00 to 3.00 mass% of Ni, 0.20 to 0.70 mass% of Cr, and 0.10 to 0.50 mass% of Mo A method for producing a high strength and high toughness cast steel material for an ingot having a composition further containing Fe, Fe and unavoidable impurities, the method comprising:
Annealing step of performing heat treatment at 1,000 to 1,000 ° C;
A quenching step of performing a heat treatment at 850 to 950 ° C .; And
A method of manufacturing a high strength and high toughness cast steel comprising a tempering step of performing a heat treatment at 610 to 670 ℃. The method according to claim 6,
A method of producing a high strength and high toughness cast steel further comprising a stress-relaxing annealing step of performing a heat treatment at less than 610 ° C. after the tempering step. The method according to claim 6 or 7,
Each of the annealing step and the quenching step comprises a cooling step, and
In all of the cooling steps, the cooling step is performed at a slower cooling rate than when cooling by liquid immersion. 9. The method according to any one of claims 6 to 8,
The ingot composition is a method for producing a high strength and high toughness cast steel, characterized in that it further satisfies at least one of the conditions containing V up to 0.05% by mass and the conditions containing 20 to 150ppm N.

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